"The SF approach: an awareness that things could have been different, that this is one of many possible worlds, that if you came to this world from some other planet, this would be a science fiction world."
- Neal Stephenson

It hunts an individual keyed to his or her perspiration. The Hound is one of the parts of the novel I remember most vividly (having read it over thirty years ago).

The mechanical Hound slept but did not sleep, lived but did not live in its gently humming, gently vibrating, softly illuminated kennel back in a dark corner of the fire house. The dim light of one in the morning, the moonlight from the open sky framed through the great window, touched here and there on the brass and copper and the steel of the faintly trembling beast. Light flickered on bits of ruby glass and on sensitive capillary hairs in the nylon-brushed nostrils of the creature that quivered gently, its eight legs spidered under it on rubber padded paws.

Nights when things got dull, which was every night, the men slid down the brass poles, and set the ticking combinations of the olfactory system of the hound and let loose rats in the fire house areaway. Three seconds later the game was done, the rat caught half across the areaway, gripped in gentle paws while a four-inch hollow steel needle plunged down from the proboscis of the hound to inject massive jolts of morphine or procaine.

The Hound is described in the chilling Bradbury style, contrasting the animal characteristics of a real hound with references to brass and steel and nylon.

The Six-Million Dollar Nose (E-Nose)

The really unique part of the Hound is already under development by NASA. Dubbed the "E-Nose" is a sophisticated electronic sensor developed to help astronauts monitor air quality aboard spacecraft. The nose is programmed to detect 12 different compounds that could spell trouble if released inside the confines of a spaceship.

"Space crews are very, very busy," said Amy Ryan, principal investigator for E-Nose at NASA's Jet Propulsion Laboratory. "Anything we can do to automate their tasks and keep the space habitat safe is highly desirable."

Besides checking for accidental chemical spills, NASA officials hope the E-Nose will be able to detect fires, a major hazard in space, before they break out. Earth-bound uses include "sniffing" for unexploded land mines, for spills in chemical plants that could contaminate workers, for plant ripeness to harvest and for possible diagnosis of disease based on odors from human perspiration and breath.

Another assassination device based on an animal model is the cobra, from Roger Zelazny's Lord of Light.